This invention relates to heat recovery systems, and in particular to a closed condensate recovery system that is applied to high temperature steam processors such as corrugation machines, plywood dryers, paper mills and other industrial and agricultural processors where steam is generated by a boiler, used in a processor and collected as condensate in traps for recycling to the boiler. In many processes where steam is used as the primary medium for providing thermal energy to the process, the spent steam is desired to be recovered as condensate for recycling the boiler system. Because feedwater has to be specially prepared, minimizing the quantity of makeup water is economically desirable. Before being fed to a boiler, makeup water must be deaerated to remove as much trapped air as possible to prevent corrosion of the heat tubes of the boiler. Furthermore, because boiler makeup water is supplied at ambient air temperatures, substantial energy is required to preheat the makeup water to the desired circulating temperature of the return water system.
However, trapped condensate from the processing equipment must be reduced in pressure to reenter the boiler feedwater system. If the condensate from the processing equipment is trapped to atmosphere, in addition to the loss of flash steam, a substantial loss in thermal energy will occur in reducing the condensate to the atmospheric temperature of boiling, 212.degree. F. Similarly, even if the trapped condensate is returned to a deaerator at 5 pounds internal pressure, significant loss of thermal energy and potential loss of flash steam will occur.
While substantial energy savings can be obtained by using heat exchanges for return of condensate to the feedwater system, for example as described in my U.S. Pat. No. 4,648,355, issued Mar. 10, 1987, entitled, HEAT EXCHANGER ARRAY FOR A STEP DOWN RETURN OF CONDENSATE, recovery can be improved by the flow system presently proposed.
In prior systems flow is conventionally regulated by pressure control valves which regulate water flows according to system pressures at various stages. Such systems are designed for optimized steady state conditions which are rarely encountered during normal operation of processing equipment. In addition to substantial variations in start up and shut down pressures and temperatures, fluctuations in steam quantity and in steam or vapor pressure during operation can result in disproportionate surges in water delivery circuits. In order to better coordinate and modulate the water return flow of the boiler feedwater, the zero flash closed condensate system of this invention was devised.
The latent energy in flashed steam is recovered in the final stage of heating the boiler feedwater in the flash condenser. Sensible heat from the return condensate is recovered in the intermediate stage of heating the boiler feedwater in the water to water heat exchanger. Finally, any residual energy from sensible or latent heat in the condensate on entry into the receiver is recovered by the receiver water. In this manner, all of the thermal energy in the higher temperature, higher pressure trapped condensate from the processing equipment is recovered.